Diagnose spurious Conceal group highlight gui Greys

Somehow something is setting my Conceal highlight with hard to read defaults. Verbose highlight Conceal shows:
ctermfg=7 ctermbg=242 guifg=LightGrey guibg=DarkGrey
With source in the sourced colorscheme, which seems reasonable except:
1) The exact same highlight shows up in 4 or 5 other colorschemes (of the 7-9 I tried).
2) Looking through the colorscheme shows nothing like that coloration.
3) Grepping for some clue as to where this is being set throughout /usr/share and local vim files, with no obvious candidates.

I can deal with this by explicitly declaring a Conceal highlight, but I'd like to undestand both why, and how to fix it more elegantly.

Thanks :-)

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